CVE-2026-21511
Published: 10 February 2026
Summary
CVE-2026-21511 is a high-severity Deserialization of Untrusted Data (CWE-502) vulnerability in Microsoft Office Long Term Servicing Channel. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Email Spoofing (T1672); ranked at the 46.8th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
Threat & Defense at a Glance
Threat & Defense Details
Likely Mitigating ControlsAI
Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.
Penetration testing supplies malicious serialized objects, detecting unsafe deserialization and supporting corrective actions.
Evaluation of untrusted data handling (deserialization testing) reveals unsafe processing, which the required remediation process addresses.
Untrusted serialized data can be deserialized and observed inside the chamber, blocking gadget-chain exploitation outside the sandbox.
Validates or rejects untrusted serialized data before deserialization occurs.
Identifies and blocks malicious code introduced through deserialization of untrusted data at system boundaries.
Integrity verification of serialized information can detect tampering before deserialization occurs.
Provenance of associated data allows detection of untrusted sources before deserialization or processing occurs.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Deserialization flaw in Outlook directly enables email spoofing attacks over the network with no user interaction required.
NVD Description
Deserialization of untrusted data in Microsoft Office Outlook allows an unauthorized attacker to perform spoofing over a network.
Deeper analysisAI
CVE-2026-21511 is a deserialization of untrusted data vulnerability (CWE-502) in Microsoft Office Outlook. Published on 2026-02-10T18:16:33.337, it carries a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N). The issue enables an unauthorized attacker to perform spoofing over a network through processing untrusted data.
The vulnerability can be exploited by any unauthorized attacker with network access, requiring low attack complexity, no privileges, and no user interaction. Successful exploitation allows the attacker to achieve spoofing, with a high impact on confidentiality but no impact on integrity or availability.
Mitigation details are available in the Microsoft Security Response Center advisory at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21511.
Details
- CWE(s)